Aberrations of optical signals occur in many systems and applications. For example, recent interest has been focused on identifying aberrations in the optics of the human eye and correcting these aberrations where possible. Identification of the aberration structure in the optical system offers the possibility of imaging surfaces beyond the aberrating structure or correcting the wavefront exiting the aberrating structure or the incident wavefront. In a report entitled “Objective Measurement of Wave Aberrations of the Human Eye With the Use of a Hartmann-Shack Wave-Front Sensor”, Liang et al., J. Opt. Soc. Am. A., volume 11, number 7, pp. 1–9, July 1994, the authors disclose the use of a Hartmann-Shack wavefront sensor to measure the wave aberrations of the human eye by sensing the wavefront emerging from the eye produced by the retinal reflection of a focused light beam on the fovea. Due to the limitations of the disclosed system, attempts to improve the system have been made and are disclosed in U.S. Pat. No. 6,095,651 to Williams et al.
The basic diagram of a Hartmann-S hack system is shown in FIGS. 1 and 2. FIG. 1 shows a schematic drawing of a beam of light reflected off the retina (incident beam and beam-shaping optics not shown). In general terms, a ray of light is projected into the eye and reflected off the retina. The reflected wavefront is monitored and spot locations are recorded by a CCD camera or other imaging device. Aberrations are quantified by the deviation of these select rays from the ideal location of rays in an aberration-free system (see FIGS. 2a and 2b).
The device presented in U.S. Pat. No. 6,095,651 projects a single (relatively large diameter) laser beam into the eye. The incident beam covers the entire pupil area (6 to 8 mm in diameter). Before entering the eye, the laser source output is collimated to form a single beam with a parallel shape. The single beam enters the eye where the human optics focus the beam on the retina. The beam is reflected from the retina and passes back through the eye. For this reason, the technique is sometimes referred to as a “double pass” method—light passes through the optics of the eye twice. The emerging beam travels through multiple lenses (imaging optics) until it finally strikes the HS lenslet array (FIG. 1). The HS lenslet array separates the beam into smaller beamlets, which are focused into spots on an imaging device (typically a CCD camera). The location or displacement of the spots is recorded by the CCD camera.
FIG. 2 illustrates an example of how a uniform wavefront is sensed by the CCD camera and how an aberrating medium changes the wavefront sensed by the CCD camera. The wavefront carries a description of how the original (incident) beam was affected by the optics of the eye. The displacement of the spots from the ideal location roughly corresponds to the aberration properties of the wavefront of the emerging laser beam. If the wavefront is not aberrated or distorted at the location of a particular HS lenslet, the corresponding spot will not be displaced. If the wavefront is highly aberrated then the corresponding spot will be displaced more. The difference between the ideal location and displaced location is the general angle or slope of the wavefront at the lenslet location. Unfortunately, aberrations in the individual lenslets may also contribute to the difference between the ideal location and the displaced location. The slope of the wavefront at many different locations allows one to fit the data to a model of the wavefront.
Previous researchers have applied the re-constructed wavefront information to a compensating device like a deformable mirror. The deformable mirror allows control of a second beam of light (e.g. a flash) so that it is corrected for the aberrating properties in the eye. This technique has demonstrated improved imaging of the retina but fails to provide precise data necessary for accurate imaging.
More recently, as described in “Laser Ray Tracing Versus Hartmann-Shack Sensor for Measuring Optical Aberrations in the Human Eye”, Moreno-Barriuso et al., J. Opt. Soc. Am. A., Vol. 17, No. 6, pp. 974–985; have described a technique for using small laser beams they called “pencils of light” to measure aberrations in the eye. They termed the technique “Laser Ray Tracing.” In the work, they used a scanning mirror to create a single, small diameter, beamlet that could be moved around the eye in sequential fashion. According to the authors, the technique consists of delivering, sequentially, a series of light pencils (nonexpanded laser beams) coming from the same point object but passing through different locations at the exit pupil plane. The trajectory of the light pencils (rays) is controlled by means of a two dimensional XY optical scanner driven by moving magnet actuators and by additional optics (collimator) when needed. Using this system, the authors were only able to process 4–5 rays per second.
Therefore, there remains a need for improved systems and method of compensating for aberrated wavefronts.